Humidity conditioning for water-based condensational growth of ultrafine particles

ABSTRACT

A particle growth apparatus includes a temperature-controlled humidifier coupled to a water-based condensation growth system. The humidifier may include a tube of sulfonated tetrafluoroethylene-based fluoropolymer-copolymer and surrounded by a region containing water or water vapor. The apparatus includes a wetted wick and wick sensor in the condensation growth system, configured such that the gas sample flows through the sulfonated tetrafluoroethylene-based fluoropolymer-copolymer tube into the condensation growth system.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/754,537 filed on Nov. 1, 2019.

GOVERNMENT RIGHTS

This invention was made with government support under contractHDTRA1-16-C-0065 awarded by the United States Department of Defense.Defense Threat Reduction Agency. The government has certain rights tothis invention.

FIELD

The technology pertains to the measurement of particles suspended in airor other gas.

BACKGROUND

For many decades particle condensation methods have been used to enablethe detection, or collection of submicrometer-sized particles suspendedin air or other gas. More recently, condensational growth has also beenapplied to aerodynamics focusing, or to enhance the electrical chargingof these ultrafine particles. Condensational growth is used becauseindividual gas-borne particles smaller than about 100 nm are difficultto detect optically and are difficult to manipulate by inertial means.Condensation has been used to enlarge particles as small as a fewnanometers, or a few tens of nanometers in diameter, to form micrometersized droplets, which are then detected optically, collected inertially,or otherwise manipulated.

For water condensation systems used in particle counters, it isadvantageous to condition the flow to a moderately high, or highrelative humidity prior to measurement.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

One general aspect includes a particle growth apparatus. The apparatusincludes a temperature-controlled humidifier coupled to a water-basedcondensation growth system. The humidifier may include a tube which maycomprise sulfonated tetrafluoroethylene-based fluoropolymer-copolymerand may be surrounded by a region containing water or water vapor. Theapparatus also includes a wetted wick and wick sensor in thecondensation growth system. The apparatus also includes the humidifierconfigured such that the gas sample flows through the sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tube into thecondensation growth system.

Another aspect of the technology is an apparatus including a humidifiercoupled to a water-based condensation growth system. The humidifier mayinclude a tube comprising sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer surrounded by a region containing wettedcrystals of polyacrylate salt. A gas sample flows through the sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tube into thecondensation growth system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a system for condensationally enlarging the particulatematter suspended air or other gas, consisting of a humidity conditionerand water condensation growth tube, the growth tube equipped with a wickto maintain wet walls, and a wick sensor to detect the level of wickwater saturation.

FIG. 2 depicts a prior art humidifier known consisting of a length ofsulfonated tetrafluoroethylene-based fluoropolymer-copolymer tubingcontained within a water bath, and optionally with a means ofcontrolling the temperature the water bath.

FIG. 3 depicts a humidity conditioner in accordance with the presenttechnology comprising a length of sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer tubing passing through a container filled withsodium polyacrylate crystals, and optionally including a containertemperature controller.

FIG. 4 depicts a humidity conditioner in accordance with the presenttechnology comprising a length of sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer tubing passing through a container that containsa water vapor-permeable pocket of sodium polyacrylate crystals, andoptionally including a container temperature controller.

FIG. 5 depicts a humidity equalizer in accordance with the presenttechnology comprising two lengths of sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tubing carrying sampleflow pass through a container, which optionally may contain water orsodium polyacrylate crystals for humidification, and that optionally mayinclude a container temperature controller.

FIG. 6 is a graph of relative humidity of the flow at an inlet andoutlet of a humidifier of the configuration of FIG. 2, with operation at20°-25° C.

FIG. 7 is a graph of the relative humidity of the flow at inlet andoutlet of a humidity conditioner containing sodium polyacrylate crystalswith operation at 20°-26° C.

FIG. 8 depicts a humidity conditioner containing sodium polyacrylatecrystals shown coupled to a water-based particle condensation systemequipped with a continuous wick, a wick sensor, a stage for internalwater vapor recovery, and a controller to regulate the temperature ofthe water recovery stage.

FIG. 9 illustrates an application of the device shown in FIG. 5.

DETAILED DESCRIPTION

The technology pertains to the measurement of particles suspended in airor other gas. More specifically, the technology pertains to devices andmethods in which the size of particles is enlarged through condensationof water vapor onto the particle. These particle condensation techniquesare most commonly applied to the detection, collection or inertialmanipulation of airborne particles that are smaller than a fewmicrometers, or a few hundred nanometers in diameter.

This technology provides a method and apparatus to readily condition thehumidity of the flow carrying the particles of interest and itsincorporation into a particle condensation system for which thecondensing fluid is water. It is of specific interest to those systemsthat recover water vapor internally, within the particle condensationalgrowth region. Such systems can be configured to provide sustainedoperation without addition of water in instances when the sampled airflow is at sufficiently high relative humidity. This technologyaddresses practical means to condition the relative humidity of thatsampled air flow to thereby allow continued, sustained operation under awide range of environmental conditions. Although application is tomeasurement of particles in air, it may also be applied to systems wherethe carrier is nitrogen, or other gas.

FIG. 1 illustrates an apparatus 100 for condensationally enlarging theparticulate matter suspended air or other gas. The system 100 comprisesof a humidity conditioner (or humidifier) 110 and water condensationgrowth tube 120. The humidity conditioner may optionally include aheater or temperature controller 146. The growth tube 120 is equippedwith a wick 130 to maintain wet walls, and a wick sensor 135 to detectthe level of wick saturation (generally with water or other fluid). Thesystem 100 enlarges the particulate matter suspended air or other gasthrough water condensation, in which the flow of suspended particulatematter is introduced into a humidity conditioner 110, and then into awater condensation growth tube 120. The growth tube 120 subjects theflow to changes in temperature in a manner that creates relativehumidity levels in excess of 100% within much of the flow, therebyinitiating the condensational growth onto the suspended particles. Thecondensation growth tube may be a laminar flow instrument, such as thewater condensation method described by Hering et al. (Hering, S. V., &Stolzenburg, M. R. (2005). A method for particle size amplification bywater condensation in a laminar, thermally diffusive flow. AerosolScience and Technology, 39(5), 428-436; Hering, S. V., Stolzenburg, M.R., Quant, F. R., Oberreit, D. R., & Keady, P. B. (2005). Alaminar-flow, water-based condensation particle counter (WCPC). AerosolScience and Technology, 39(7), 659-672; and Hering, S. V., Lewis, G. S.,Spielman, S. R., & Eiguren-Fernandez, A. (2019). A MAGIC concept forself-sustained, water-based, ultrafine particle counting. AerosolScience and Technology, 53(1), 63-72. (hereinafter Hering et al.(2019)), or the flexible working fluid method described in German PatentDE 10 2005 001 992 A1, or the stream-wise gradient method described inU.S. Pat. No. 7,656,510 and (Roberts, G. C., & Nenes, A. (2005). Acontinuous-flow streamwise thermal-gradient CCN chamber for atmosphericmeasurements. Aerosol Science and Technology, 39(3), 206-221.). It couldalso be an adiabatic expansion type instrument, such as described bySkala, G. F. (1963) (A New Instrument for the Continuous Measurement ofCondensation Nuclei. Analytical Chemistry, 35(6), 702-706), or a mixingtype instrument such as described by U.S. Pat. No. 8,465,791. All ofthese water-based condensation growth systems work by creatingsupersaturated conditions, defined as regions in which the relativehumidity exceeds 100%. The higher the supersaturation level, the smallerthe particle that is activated to grow through uptake of water vapor.

Each of these water-based condensation growth systems would be aided byhaving known, high relative humidity at the growth tube inlet. For some,the humidity conditioning also enables sustained operating without theneed to provide liquid water to the growth tube itself. Manycondensation systems are equipped with a wick that holds the liquidwhich must be evaporated to create the supersaturation. Some of thesealso have a region of water vapor recovery. As described by Hering et al(2019), incorporation of a water vapor recovery region enables thecondensation growth tube to operate without consuming water, providedthe sampled flow is sufficiently humid. Thus, coupling to a humidityconditioner at the inlet, and with appropriate feedback control, enablessustained operation, without the need to add water to the growth tubeitself.

With reference to FIG. 1, a flow 142 containing suspended particlesenters the humidity conditioner 110 at the inlet 140, where it isbrought to a desired humidity and temperature. The flow is thenintroduced into a laminar flow type particle condensation growth tube120. The growth tube has a wick 130 that lines interior walls throughwhich the flow passes. The growth tube 120 also has two or moretemperature regions. When the wick is wet, the warmer regions evaporatewater into the otherwise colder flow, creating a region ofsupersaturation, and thereby activating the condensational growth of thesuspended particles. The flow exits the growth tube at the output,carrying condensationally enlarged particles. These condensationallyenlarged particles are droplets, much as in a cloud, as they form aroundindividual particles in the flow.

For proper operation of the system of FIG. 1, the wick should be wet,but not flooded, i.e. it must be able to absorb water vapor thatcondenses on its surface. FIG. 1 shows a wick sensor 135, located withinthe interior of the growth tube 120, which detects the saturation levelof the wick 130. This wick sensor provides a feedback signal 125 whichis used to adjust the operating conditions of the humidity conditioner110 and growth tube 120 to maintain the wick 130 at the targetsaturation level. Such adjustment in operating conditions can be throughcontrolling the temperature of the humidity conditioner, or of the watervapor recovery region of the growth tube, or both, using amicroprocessor controller 160. If the wick sensor indicates that thewick is drying, the temperature of the humidifier can be increased toprovide more humidity into the growth tube. The thermistor 144 measuresthe temperature inside the humidifier, and the microprocessor 160controls the heater 146 to achieve the degree of humidification requiredto keep the wick moist, as indicated by the wick sensor. The output flow142 of the growth tube 120 may be provided to a particle counter.

FIG. 2 shows a humidifier using a sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer consisting of a length of sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tubing containedwithin a water bath, and optionally with a temperature controller forthe water bath.

FIG. 2 illustrates a commonly used approach of humidifying a gas stream.

The air or gas flow to be humidified is passed through a length ofsulfonated tetrafluoroethylene-based fluoropolymer-copolymer (commonlysold as Nafion™ tubing from PermaPure, LLC (www.permapure.com, Lakewood,N.J.)) that is housed in a bath of water. Alternatively, the tubing maybe surrounded by a humid flow. The sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer is a co-polymer that is highly selective in thetransport of water. The sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer membrane absorbs water from region of high-watercontent and releases this water to the region of low humidity.

More specifically the tubing is a copolymer of tetrafluoroethylene(Teflon®) and perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid thatwas developed by DuPont. The tubing used is highly resistant to chemicalattack, but the presence of its exposed sulfonic acid groups providesthe water absorption characteristics. Sulfonic acid has a very highwater-of-hydration, absorbing 13 molecules of water for every sulfonicacid group in the polymer; consequently, the tubing can absorb 22% byweight of water. When one side of the sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer membrane is in contactwith a humid flow or with liquid water, the sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer becomes hydrated.Interconnections between the sulfonic acid groups lead to very rapidtransfer of water across the sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer membrane. The extent of hydration at the surfaceof the sulfonated tetrafluoroethylene-based fluoropolymer-copolymerdepends on the water content of the space it adjoins, and thus thehydrated sulfonated tetrafluoroethylene-based fluoropolymer-copolymerwill then release the hydrated water to the less humid flow on theopposite side of the membrane. This property makes sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer suitable for eitherhumidification or dehumidification. Because the hydration process isessentially a chemical reaction, indeed is described as a first orderkinetic reaction, the rate of water vapor transport depends not only onthe relative humidity of the flow on either side of the sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer membrane, but also onits temperature. This material is manufactured by PermaPure LLC, andsold under the trade name Nafion.

In another alternative, rather than using humid air flow or water bath,a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer tube isrun through a bed of sodium polyacrylate crystals. FIG. 3 depicts ahumidity conditioner in accordance with the present technologycomprising a length of sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer tubing passing through a container filled withsodium polyacrylate crystals, and optionally including a containertemperature controller. The tubing can pass directly through the sodiumpolyacrylate, as illustrated in FIG. 3.

Alternatively, the sodium polyacrylate crystals can be contained in awater vapor-permeable pocket that is housed within the humidifier, asshown in FIG. 4. FIG. 4 depicts a humidity conditioner in accordancewith the present technology comprising a length of sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tubing passing througha container that contains a water vapor-permeable pocket of sodiumpolyacrylate crystals, and optionally including a container temperaturecontroller. The sodium polyacrylate is a super-absorbent material,capable of taking up to as much as 30 times is weight in water. It isavailable commercially for a variety of applications. It serves as awater absorbent in diapers and pet pads, as well as a water source forplants or for cigar humidors. It has the chemical property that itreleases or absorbs water vapor to maintain a relative humidity near 80%in the air space that surrounds the crystals. When the desired targetfor humidification is near 80%, this is a better option than the liquidwater bath illustrated in FIG. 1, which will tend to bring the flow tonear 100%. It has the further advantage that there is no liquid thatcould spill.

FIG. 5 shows a “humidity equalizer”, and shows separate flows 142 a, 142b each contained in its own sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer tube 140 a, 140 b, passing through a commonhumidity conditioner. Two flow paths are shown, but multiple flow pathsare possible. This configuration applies to dual, or multichannel,condensation particle counters which make simultaneous measurements intwo or more flows. In some applications, such as simultaneous indoor andoutdoor sampling, the relative humidity of the different channels mayvary. As the long-term performance of water-based condensation particlecounters may depends on the humidity of the sampled air flow, thehumidity equalization ensures equal performance among the channels.

One application is dual channel system 502 a for assessing particleconcentrations inside, and immediately outside, a respirator mask whileit is being worn. This is illustrated in FIG. 9. An important aspect inthe performance of a respirator is how well it fits on the subject'sface, that is whether or not air leaks around the respirator, ratherthan passing through its filters. A standard test of how well arespirator fits the subject is to compare the concentration of particlesinside the mask to that outside the mask. The measurement inside themask is done by sampling through the ‘drink tube’ provided in suchrespirators, while the ambient measurement is made through a tubeclipped near the subject's breathing zone. If the particle counting isdone is with an instrument such as a condensation particle counter whichis sensitive to ultrafine particles, then the natural abundance ofparticles in the air is adequate to provide sufficient statistics forthis test. However, ambient particle concentrations are quite variablein time. Thus, it is best to have simultaneous measurements inside, andoutside, of the mask while it is being worn. Water-based condensationparticle counter systems, which are environmentally friendlier thanalcohol-based condensation particle counters, can be affected by thedifference in the humidity between the two flows. The flow from insidethe mask will be humidified from the wearer's breath and will be muchhigher than the relative humidity for the line sampling the ambient airoutside the mask, except for very humid environments. The humidityequalizer 502 a exchanges water vapor between the two lines whilemaintaining the particle concentrations in each line, facilitating morebalanced and equal measurements between the two lines. As illustrated inFIG. 9, this can be accomplished by passing the two flows through ahumidity equalizer 502 a consisting of sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tube housed in a smallchamber (or pocket) 802 (similar to FIG. 4) containing wetted,polyacrylate crystals. Experiments show that if each channel has a flowof 100 cm³/min, a length of 70 mm of sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer housed with thewetted, polyacrylate crystals is adequate to equalize, within 10%, therelative humidity between the two channels. There are several advantagesif the polyacrylate crystals are contained within a small netted pocket802, permeable to water vapor. First, no liquid water is required, andthus the system is motion tolerant. Second, the system can be designedsuch that the pocket of polyacrylate crystals is readily removed,rejuvenated by soaking in water, and returned to the humidifier.

FIG. 6 shows data on the performance of a standard-type humidifier ofthe configuration of FIG. 2. Relative humidity of the flow at inlet andoutlet are shown for flow passing through a sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tube submerged in awater bath, with operational temperatures of 20°-25° C. Active length is30 mm or 60 mm, as indicated by the legend, and flow is 300 cm³/min.This is a higher flow, in proportion to the length of the tubing, forfull humidification. Yet the approach significantly increases therelative humidity for dry flows. The output relative humidity is alwaysabove 50% for the shorter tube length, and above 80% for the longerlength. At input relative humidity above 90% the change in relativehumidity is small. As is characteristic of convective diffusion in alaminar flow in a tube, the extent of water vapor transport depends onthe ratio of the length of the tube to the flow rate. For the data ofFIG. 6 we find that the 30 mm length at 300 cm³/min flow, or a length toflow rate ratio of 0.6 s/cm², provides sufficient humidification attemperatures of 20°-25° C. A humidifier with twice this length, suchthat the ratio of length to flow rate is 1.2 s/cm², gives uniformly highrelative humidity across a wide range of input relative values.

FIG. 7 shows data from the humidity conditioner of this invention, whichuses sodium polyacrylate crystals in place of the water bath. Beforeplacing in the humidifier, these crystals are wetted with water, suchthat the water molecules absorb onto the crystals. For the configurationof FIG. 3, with the sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer running through the crystals, and with operationat room temperature, the exiting relative humidity is in the range of70% to 80%, regardless of the input relative humidity. When thesulfonated tetrafluoroethylene-based fluoropolymer-copolymer tube isplaced in the airspace surrounding the crystals, the conditionerresponds a bit more like the water bath system of FIG. 6.

We find that if a flow of 100 cm³/min passes through a 70 mm length ofsulfonated tetrafluoroethylene-based fluoropolymer-copolymer tubingsurrounded by wetted sodium polyacrylate salt, the relative humidity ofthe output flow is approximately 80%, regardless of whether the humidityat the input was 10% or 90%. The corresponding ratio of the length ofthe tube (7 cm) to the flow rate (100 cm/min=1.7 cm/s) is 4 s/cm². Aconvenient feature of using the sodium polyacrylate is that it is easilycontained within a small water vapor-permeable pocket that is readilyrejuvenated. This pocket becomes the bed of polyacrylate crystals shownin FIG. 4. This pocket can be readily removed from the humidifier,rejuvenated by wetting with, or soaking in, water, and then returned tothe humidifier. The data of FIG. 7 labeled “Nafion running next tocrystals” was obtained with just such configuration. The polyacrylatewas contained within a pocket comprised of a fine net that preventedescape of the crystals, which are several tenths of millimeters in size,and yet is completely permeable to water vapor. Because there is noliquid water in the system, it does not have the potential of leaking asdoes the water-bath configuration of FIG. 2.

Other salts of polyacrylate could be employed equally well, but thesodium polyacrylate is a commonly used water absorbent. Like many salts,the polyacrylate has a water vapor equilibrium value that is less than100%. In the case of polyacrylate, this value is near 80%. Polyacrylatehas the further advantage that it can absorb large amounts of water,while maintaining its crystalline form. This offers the practicaladvantage that water is stored without the potential for leaking ofliquid water.

FIG. 8 is another alternative embodiment of the technology. FIG. 8depicts a humidity conditioner 810 containing sodium polyacrylatecrystals shown coupled to a water-based particle condensation system 820equipped with a continuous wick, a wick sensor, a stage for internalwater vapor recovery, and a controller to regulate the temperature ofthe water recovery stage. FIG. 8 also shows temperature control of thehumidifier by means of the heater 146, controlled by the microprocessorto reach a desired temperature as measured by the temperature sensor144. From experimental data, this is not needed if operations are attypical indoor temperatures (18-30° C.), but may be useful for operationin environments that are very cold.

FIG. 8 shows the humidity conditioner 810 containing a pocket 802 ofpolyacrylate crystals (similar to that of FIG. 4) coupled to theself-sustaining water condensation growth tube 820 of the type describedby Hering et al. (2019). The growth tube 820 has three temperaturestages 822, 824, 826 (each represented by different cross-hatching inthe figure), lined by a continuous wick 130. The first stage 822 hascooled walls, typically around 5-15° C., and both cools the flow andcaptures water vapor from the sampled air flow 142 onto the wick. Thesecond stage 824 is warm, typically 30° C. to 40° C. warmer than thefirst stage. Water evaporates from the wet wick walls in this stage, anddiffuses into the cooler, entering flow, thereby creating the watervapor supersaturation necessary to activate the condensational growth ofsmall particle suspended in the flow. The third stage 826 is againcooled and recovers water vapor from the flow into the wick. Therecovered water is transported to the warm, second stage via capillaryaction. Within the third stage condensational growth continues as thesupersaturation is maintained because both the temperature and watervapor decrease at similar rates. At the exit of the growth tube the flowis cold, close to the temperature of the walls of the third stage, butit is still saturated, or somewhat supersaturated, and contains dropletsformed around individual particles in the sampled air flow.

Hering et al. (2019) explored the relationship between the water contentof the flow exiting a three-stage growth tube such as that shown in FIG.8, and the growth tube operating temperatures. It was found that theabsolute humidity of the exiting, droplet-laden flow could be as low as8° C., or as high as 20° C., without compromising the efficiency ofdroplet formation. Thus, provided that the dew point of the sampled airflow at the inlet of the growth tube is above about 8° C., it ispossible to operate the system in a manner in which the net waterconsumption is near zero. Essentially, enough water vapor can becaptured to maintain a wetted wick. The 8° C. dew point corresponds to arelative humidity greater than 40% at 22° C. Prevailing ambient absolutehumidity values are often less than this. Yet, when coupled to thehumidity conditioner, the system could sustain operation with respect tothe water consumption, for long periods of time.

FIG. 8 also shows a wick sensor 135, which detects the local watersaturation level of the wick. This provides a feedback signal to controlthe extent of water recovery in by the wick and facilitates maintenanceof the wick at the desired water saturation level. For proper operationthe wick must be sufficiently wet to generate the high supersaturationrequired for particle activation and growth, yet not so wet that iscannot take up water vapor that condenses on its surface. Were water tocondense onto the wick, and not be absorbed by the wick, condensationdroplets form that can eventually lead false counts, and even toflooding of the optics when used as a condensation particle counter.Thus, proper saturation level within the wick is important parameter tocondensational growth performance. To maintain the proper saturationlevel, a microprocessor uses the signal from the wick sensor to set thetemperature of the third, water recovery stage of the growth tube, andoptionally also of the humidity conditioner.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

We claim:
 1. An apparatus, comprising: a temperature-controlledhumidifier coupled to a water-based condensation growth system, thehumidifier comprising a tube comprising sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer and being surroundedby a region containing water or water vapor; a wetted wick and wicksensor in the condensation growth system; and the humidifier configuredsuch that the gas sample flows through the sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tube into thecondensation growth system.
 2. The apparatus of claim 1 wherein a lengthof the tube is between 30 and 60 mm.
 3. The apparatus of claim 1 whereinthe ratio of the length of the tube to a volumetric flow rate of air orother gas through the tube is greater than 0.6 s/cm².
 4. The apparatusof claim 1 wherein the region surrounding the tube is liquid water. 5.The apparatus of claim 1 wherein the region surrounding the tube iswater vapor and is provided by wetted crystals of a polyacrylate salt.6. The apparatus of claim 1 wherein the temperature of the humidifiercontrolled based on the reading of the wick sensor.
 7. The apparatus ofclaim 4 wherein the temperature of the humidifier is controlled towithin the range of 20-30° C.
 8. The humidifier of claim 1 containingtwo or more tubes to provide equal humidity conditioning of separate airor gas flows.
 9. An apparatus, comprising: a humidifier coupled to awater-based condensation growth system, the humidifier comprising a tubecomprising sulfonated tetrafluoroethylene-based fluoropolymer-copolymersurrounded by a region containing wetted crystals of polyacrylate salt;wherein a gas sample flows through the sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer tube into thecondensation growth system.
 10. The apparatus of claim 9 wherein thelength of the tube is about 70 mm.
 11. The apparatus of claim 9 whereina ratio of a length of the tube to the volumetric flow rate of air orother gas through the tube is more than 4 s/cm².
 12. The apparatus ofclaim 9 wherein the temperature of the humidifier is controlled towithin the range of 20-30° C.
 13. The humidifier of claim 9 thepolyacrylate salt crystals are contained within a water vapor-permeablepocket.
 14. The humidifier of claim 13 the polyacrylate crystals arecontained within a water vapor-permeable pocket that is readily removedto facilitate wetting of the crystals.
 15. The humidifier of claim 9containing two or more tubes to provide equal humidity conditioning ofseparate air or gas flows.
 16. The humidifier of claim 15 configured forconditioning of air flows for a dual channel condensation particlecounter sampling simultaneously from inside and outside of a respiratormask.
 17. A method, comprising: surrounding a tube comprising sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer with water or watervapor; passing a particle laden flow through the tube to an output ofthe tube; controlling the temperature of the tube; passing the flow fromthe output of the tube to a channel including a wetted wick comprising acondensation growth system to an output of the channel.
 18. The methodof claim 17 wherein the ratio of the length of the tube to a volumetricflow rate of air or other gas through the tube is greater than 4 s/cm².19. The method of claim 17 wherein the surrounding the tube comprisessurrounding the tube with liquid water.
 20. The method of claim 17wherein the surrounding the tube comprises surrounding the tube withwater vapor using wetted crystals of a polyacrylate salt.
 21. The methodof claim 17 wherein the controlling the temperature is based on areading of the wick sensor.
 22. The method of claim 17 wherein thecontrolling the comprises controlling within the range of 20-30° C. 23.The method of claim 17 wherein the controlling wherein the surroundingcomprises surrounding two or more tubes to provide equal humidityconditioning of separate air or gas flows.